The present invention relates in general to a process for producing a planar heterostructure for the fabrication of electronic and/or optoelectronic devices, in particular in the microelectronics industry.
One of the crucial and often limiting steps of processes for fabricating integrated circuits is the step of etching by photolithography. In order to produce very thin patterns, typically with a width of less than 100 nm, photolithographic techniques require equipment which is both very expensive and time-consuming, such as optical exposure using x-rays or direct writing with an electron beam.
It would therefore be desirable, as far as possible, to do without the photolithography step for the fabrication of such microelectronic devices, and in particular for optoelectronic devices.
It has now been found that it is possible to do without or circumvent the photolithographic step for producing patterns in active material (CMOS gate or multiple quantum well layer, for example) with a very thin predetermined width by using the very good control which there is over the thickness of the deposits. Such an approach makes it possible to produce (principally optoelectronic) devices which are planar, that is to say at the surface of the specimen rather than in its bulk.
According to the invention, the process for fabricating a planar heterostructure comprises:
a) etching, in a semiconductor substrate and starting from an upper main surface of the substrate, at least one trench with predetermined width and depth having a bottom wall and vertical side walls;
b) depositing, on the upper main surface of the substrate and the bottom and side walls of the trench, a stack of successive and alternate layers of Si1xe2x88x92Gex (0 less than xxe2x89xa61) and Si, the number and the thickness of which depend on the final use intended for the heterostructure; and
c) chemical-mechanical polishing in order to obtain a final heterostructure having a plane upper main surface, level with which the stack layers deposited in the trench are flush.
In a first embodiment of the process of the invention, the stack of successive alternate layers does not fill the trench completely, and before the chemical-mechanical polishing step, a layer of a dielectric material is deposited in order to complete the filling of the trench.
In a second embodiment of the process of the invention, the stack of successive alternate layers in the trench is produced so as to leave a, for example central, recess in the trench, and before the chemical-mechanical polishing, the stack is etched so as to extend the recess down into the material of the substrate. A dielectric material is then deposited in order to fill the extended recess, and the chemical-mechanical polishing is carried out in order to obtain the final planar heterostructure.
In a third embodiment of the process of the invention, prior to the deposition of the stack, a layer of dielectric material filling the trench is deposited in the latter, and the dielectric material is etched so as to form, in the trench, a, for example central, partition dividing the trench into two half-trenches, and the two half-trenches are filled by depositing a stack of successive alternate layers.
The substrates suitable for the present invention are substrates made of bulk or thin-film silicon, for example silicon-on-insulator (SOI), or of bulk or thin-film germanium.
Clearly, the process of the invention may involve the production of a plurality of trenches in the substrate, according to the application intended for the device. The trenches may have identical or different widths depending on the a application. The depth of the trenches will also vary according to the application.
The side walls (flanks) of the trenches are preferably vertical, that is to say as close as possible to the  less than 100 greater than  crystallographic direction and generally at an angle of from 70xc2x0 to 80xc2x0 with respect to the plane of the upper main surface of the substrate. If the flanks of the trenches are not vertical, then layers with uniform thickness will not be obtained on the flanks, tops and bottoms of the trenches during the subsequent deposition of the stack. Furthermore, the corner regions at the bottom or at the top of the trenches are sensitive regions in so far as the growth of the layers of the stack in these regions, although conformal, is perturbed. It is therefore preferable to distance these regions from the future active regions, for example by producing deeper trenches, that is to say having a high aspect ratio d/w (d=depth and w=width). The flanks of the trenches will also preferably have monocrystalline quality, that is to say that, after the etching, the surfaces of the trench will be oxidized then the oxide layer will be removed, for example by dissolving, in order to obtain surfaces without defects.
The trenches may be produced by any conventional process such as photolitho-etching using a mask.
The deposition of the alternate thin layers of Si1xe2x88x92Gex and Si is in general heteroepitaxial or epitaxial deposition involving chemical vapour deposition (CVD). If the starting substrate is silicon or silicon-on-insulator, the Ge concentration in the Si1xe2x88x92xGex alloy immediately deposited on the substrate may vary from a few per cent to 100% atomic. Nevertheless, the thickness of the layer will be such that this layer is constrained on the silicon substrate, that is to say less than the critical thickness beyond which the layer relaxes to give out dislocations. And vice versa in the case of depositing Si-rich Si1xe2x88x92Gex on a germanium substrate. The various layers are deposited under conditions of temperature, pressure and active gas-species flux allowing conformity of the deposit to be obtained, that is to say a constant thickness of each film over the surface, the flanks and the bottom of the trench. This is important because the final result in terms of xe2x80x9cline widthxe2x80x9d will depend on the precision of the thickness of the films on the flanks of the trench. These deposits may be produced using a conventional industrial single-wafer epitaxy machine, which proves particularly suitable for forming the desired Si1xe2x88x92Gex /Si multiple layers. The deposits used will preferably be in the surface regime, that is to say a deposition regime in which it is the surface coverage which controls the progress of the chemical reactions (as opposed to the diffusion regime). Hence, with precursor gases such as SiH4, GeH4 and H2 (the latter also playing the part of a carrier gas), deposition temperatures varying from 450 to 700xc2x0 C. may be used. In order to obtain a conformal deposit, especially with high aspect ratios, it is preferable to carry out the deposition with a low total pressure, typically of the order of 2.6 kPa (20 torr). However, higher pressures ranging up to atmospheric pressure may be used.
For certain applications, a layer of a planarizing or non-planarizing dielectric material, such as SiO2 or Si3N4, may be deposited conventionally, for example by CVD, on the structure with filled trenches.
Optionally, the layers grown epitaxially or heteroepitaxially in the trenches may be doped with an n- or p-type dopant depending on the desired architecture.
After the various layers have been deposited, the chemical-mechanical polishing (CMP) is carried out in order to make the layers flush with the surface of the substrate. Any conventional CMP process may be used, the conditions being determined so as to obtain a plane surface whatever the architecture.
Lastly, for yet other applications, layers of materials such as Si, SiGe or dielectrics may be re-deposited on the polished surface after CMP, for example by conventional epitaxy.